Exodus 4:20-23
English Standard Version
21And the Lord said to Moses, "When you go back to Egypt, see that you do before Pharaoh all the miracles that I have put in your power. But I will harden his heart, so that he will not let the people go.
22Then you shall say to Pharaoh, ‘Thus says the Lord, Israel is my firstborn son,
23and I say to you, "Let my son go that he may serve me." If you refuse to let him go, behold, I will kill your firstborn son.’"
